Yes, some of the precipitate may be undissolved calcium hydroxide which is reusable next time, and some is unusable calcium carbonate and magnesium hydroxide.
I discuss it here:
What is that Precipitate in My Reef Aquarium?
http://reefkeeping.com/issues/2005-07/rhf/index.htm
from it:
http://reefkeeping.com/issues/2005-07/rhf/index.php#9
Precipitates on the Bottom of Limewater Reservoirs
The solids on the bottom of a limewater reservoir (Figures 5 and 7) contain everything that did not dissolve, or that dissolved and later precipitated from solution. Such solids could contain magnesium hydroxide and carbonate, calcium hydroxide and carbonate and a variety of other impurities such as copper salts, alumina, silica, etc.
In order to determine what is in these deposits, I tested a sample of the white solid material that had been collecting for months on the bottom of my limewater reservoir, and detailed the results in a previous article. I removed the white sludge along with some limewater. The mixture of solid and liquid was acidified to dissolve it, and it was tested for calcium, magnesium, and strontium. The results are shown in Table 1. Only relative concentrations are shown as no effort was made to dry the sample prior to analysis, making absolute concentrations meaningless.
As anticipated, based on the very low solubility of magnesium hydroxide and the high concentration of hydroxide ion in limewater, the solid material on the bottom of the limewater is enriched with magnesium. Relative to calcium, magnesium is enriched by a factor of 13 in the sludge compared to the solid starting quicklime. This magnesium may be present as both magnesium hydroxide and magnesium carbonate, but because magnesium carbonate is fairly soluble compared to calcium carbonate, it is most likely that the primary magnesium salt is magnesium hydroxide. It may also be mixed calcium and magnesium carbonates.
Interestingly, strontium is actually depleted by a factor of two relative to solid starting quicklime, indicating that it is less likely than calcium to end up on the bottom of the reservoir. While strontium carbonate is somewhat less soluble than calcium carbonate, the strontium concentration in the limewater is so low that SrCO3 may not actually be saturated, so it may not precipitate at all. The strontium that is there may simply be copreciptiated with calcium carbonate.
The solids on the bottom of a limewater reservoir or the residue left in a limewater reactor can also contain other materials. Phosphate, for example, would be insoluble in limewater, precipitating as calcium phosphate. Many toxic metals, such as copper, are also insoluble in the high pH of limewater, forming carbonates or oxides. These metals can also bind directly to undissolved lime or to calcium carbonate precipitates, as I showed in a previous article. In a sense, this precipitation can purify the limewater so that in some cases it may be even purer than the starting water or lime.
This purification is also seen in practice by many aquarists who have noticed the solids on the bottom of their limewater containers discolor, often to a bluish/green color suggesting copper. For these reasons, I recommend that lime solids not be dosed to aquaria when it is possible to avoid it. Letting the limewater settle for a few hours to overnight will permit most of the large particles to settle out, and whether it looks clear at that point or not, it is likely fine to use. In general, it is a good practice to leave residual solids on the bottom of limewater reservoirs rather than cleaning them out every time, as they may actually help purify the water by these precipitation mechanisms. Once the solids discolor, or have been collecting for 6-12 months, however, they should be discarded.
Figure 7. A view of the side of my limewater reservoir after the water level has lowered. The precipitate is most likely calcium carbonate formed when residual limewater on the sides reacts with air, but may also include solids that settled from the initial mixing, and so may resemble the material found on the bottom of the container (Figure 5). I can't recall ever cleaning off the sides of this container.
